1. Field of the Invention
The present invention relates to electrical connections of two printed circuit boards surface mounted on opposite sides of a mid-plane printed circuit board at angles to each other. Specifically, the invention is a routing scheme providing a direct electrical path between electrical connections.
2. Description of the Prior Art
Modern electronic devices such as computers must be able to perform complex functions both quickly and effectively. Thus, it is essential that the internal hardware of these electrical devices is connected in an efficient manner. A common hardware assembly technique involves two printed circuit boards, commonly referred to as daughter boards, that are connected to opposite sides of a mid-plane printed circuit board, commonly referred to as a mother board. These daughter boards are often connected at angles to each other. For the electrical device to function as desired, the electrical components on one daughter board must be able to transmit and receive electrical signals to and from the electrical components on the other daughter board.
There are several devices known in the prior art that provide such an electrical connection between two daughter boards. For example, Petit et al. describe in U.S. Pat. No. 4,703,394 a system for interconnecting orthogonally disposed printed circuit boards. This system involves the use of four multiway connectors. Two of the connectors are mounted on respective facing edges of the connector boards, and two of the connectors are mounted on opposite sides of the mid-plane board. The connectors mounted on the connector boards plug into the connectors mounted on the mid-plane board.
The interconnection system described in the Petit et al. requires the use of bulky connectors that makes its implementation undesirable in modern electrical devices which require a compact design.
Sample et al. describe in U.S. Pat. No. 5,887,158 a switching mid-plane and interconnecting system for interconnecting large numbers of signals. This system contains a mid-plane printed circuit board having a plurality of first connectors oriented in a first direction on one side of the mid-plane for making connections to the plurality of first printed-circuit boards. The mid-plane printed-circuit board also has a plurality of second connectors oriented in a second direction orthogonal to the plurality of first connectors on the other side of the mid-plane. The connectors are positioned such that connection pins on the plurality of first connectors and plurality of second connectors in regions of intersection are double-ended pins common to both. The remaining connection pins of the plurality of first connectors are single-ended connection pins which are connected to the single-ended connection pins of the plurality of second connectors via conductive traces on the mid-plane printed-circuit board.
The interconnection system described in the Sample et al. is subject to two important drawbacks relating to the connection of the single ended connection pins. First, the electrical path connecting the single ended connection pins may vary in length. If the contacts are used for differential signals, this presents a problem because a longer electrical path may result in signals which are out of phase. Additionally, the electrical path connecting the single ended pins may run through the mother board diagonally rather than straight (at an angle perpendicular to the top and bottom side of the mother board). This diagonal electrical path presents a problem because it results in the entrance and exit of the electrical path into and out of the mother board to be misaligned, thereby creating the need for additional drilling in the mother board and further raising the complexity of the design.
Thus, there is a need for a connection system that provides a routing scheme in which the electrical paths between connections in each pair of connections are of equal length. Furthermore, there is a need for a connection system that provides a routing scheme in which the electrical path runs straight through the mother board so that the entrance and exit of the electrical path into and out of the mother board directly overlay one another. Additionally, it is desired that this connection system is a compact system that does not require the use of bulky connector plug-ins. A connection system with these and other improve characteristics is desired.
The present invention is an electrical connection system that includes two connector printed circuit boards surface mounted on opposite sides of a mid-plane printed circuit board at angles to each other. Each of the connector boards contains pairs of electrical connections. An electrical path connects each electrical connection on the first connector board to a corresponding electrical connection on the second connector board. This electrical path consists of three elements. The first element is preferably an electrically conductive surface mount pad connecting each electrical connection on the first connector board to a via hole on the first face of the mid-plane board. The second element is preferably a mid-plane electrical trace through two overlaying via holes on each face of the mid-plane board. This connection is straight through the mid-plane board at an angle perpendicular to each face of the board. The third element is preferably another electrically conductive surface mount pad connecting the via holes on the second face of the mid-plane board to the corresponding electrical connection on the second connector board.
The length of the mid-plane electrical trace is preferably equal to the width of the mid-plane board. However, the length of the surface mount pads may vary provided that the total length of the electrical path connecting the first connections in each pair of electrical connections is equal to the total length of the electrical path connecting the second connections in the pair.
In a preferred embodiment, each via hole in each pair is fixed on an axis perpendicular to an axis that bisects the angle of misalignment of the connector boards. Additionally, each via hole in each pair may be equidistant from a point on an axis that bisects the angle of misalignment of the connector boards. For the electrical paths connecting the first connections in each of the pairs of electrical connections, the first element will be a long surface mount pad and the third element will be a short surface mount pad. Conversely, for the electrical paths connecting the second connections in each of the pairs of electrical connections, the first element will be a short surface mount pad and the third element will be a long surface mount pad. Thus, the total length of the electrical path connecting the first connections in each pair of electrical connections is equal to the total length of the electrical path connecting the second connections in the pair.
In a another preferred embodiment, each via hole in each pair is fixed directly on an axis that bisects the angle of misalignment of the connector boards. In this embodiment, all surface mount pads are equal in length. Thus, the total length of the electrical path connecting the first connections in each pair of electrical connections is equal to the total length of the electrical path connecting the second connections in the pair.